Plating solution of palladium alloy and method for plating using the same

ABSTRACT

The present invention relates to a plating solution of palladium alloy and has an object to provide a plating solution of palladium alloy highly stable and capable of stably forming a plated film of a given alloy composition. The present invention relates to a plating solution of palladium alloy containing a palladium complex and a metal salt, wherein the palladium complex is coordinated with at least one neutral amino acid selected from the group consisting of glycine, alanine, valine, leucine, serine, threonine, asparagine, glutamine and tyrosine as a ligand.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plating solution of palladium alloyand a method for plating using the same.

2. Description of the Related Art

A plated palladium alloy has been used for dental and decorativepurposes, catalysts, electrical contacts, and so forth. It is also usedfor hydrogen separation membranes, because it selectively absorbshydrogen and allows it to permeate. It is known that when used for ahydrogen separation membrane, palladium alone tends to suffer hydrogenembrittlement, and is alloyed with Ag, Au or Cu to improve durabilitywhile keeping hydrogen permeation capacity for plating.

U.S. Pat. No. 4,048,023 discloses a plating solution of Pd—Au alloycomprising an amine-containing palladium chloride and a gold salt ofsulfurous acid. Japanese Patent Laid-Open No. 2005-256129 discloses aplating solution of Pd—Ag alloy which contains palladium and silver ionsand at least one of nitrate and sulfate ions, and is adjusted for its pHlevel with ammonia. Japanese Patent Laid-Open No. 2000-303199 disclosesa plating solution of Pd—Cu alloy which contains a soluble palladiumsalt, a soluble copper salt, an electroconductive compound, a compoundcontaining a pyridine ring and a soluble semi-metallic compound.

The plating solution containing ammonia, e.g., that disclosed by U.S.Pat. No. 4,048,023 or Japanese Patent Laid-Open No. 2005-256129, tendsto be difficult to keep its pH level because of evaporation of ammoniaor the like during the plating work. Palladium or the like may separateout when the plating solution has an unstable pH level, tending to varythe alloy composition in the plated film.

The plating solution disclosed by Japanese Patent Laid-Open No.2005-256129 may cause evolution of explosive fulminating silver by thereactions of ammonia with the nitrate or silver ions. The platingsolution disclosed by Japanese Patent Laid-Open No. 2000-303199, whichcontains selenium or the like as a soluble semi-metal, may show toxicityto deteriorate working environments for plating.

As discussed above, the plating solutions of palladium alloy disclosedby the patent documents are difficult to satisfy all of the suitablesolution conditions, because they may cause problems, e.g., unstablesolution conditions to separate the component(s), difficulty in forminga plated film at a stable eutectoid rate, uneven plated film surfaces,or deteriorated plating environments.

It is an object of the present invention to provide a plating solutionof palladium alloy highly stable and capable of stably forming a platedfilm of a predetermined alloy composition.

SUMMARY OF THE INVENTION

The present invention, which can solve the above problems, relates to aplating solution of palladium alloy containing a palladium complex and ametal salt, wherein the palladium complex is coordinated with at leastone neutral amino acid selected from the group consisting of glycine,alanine, valine, leucine, serine, threonine, asparagine, glutamine andtyrosine as a ligand.

In the plating solution of the present invention, the neutral amino acidas a ligand for the palladium complex works to suppress variation of pHlevel of the plating solution, controlling separation of precipitate orthe like. It is therefore possible to deposit a uniform plated filmhaving a constant palladium/metal salt ratio. A uniform plated film hasa high mechanical strength and is suitable for hydrogen separationmembranes. Moreover, the plating solution has a low environmental load,because of lack of toxic compounds, e.g., cyan.

Generally known neutral acids include, in addition to those describedabove, cystine, methionine, isoleucine, phenylalanine, tryptophan and soforth. However, those containing sulfur, e.g., cystine and methionine,may give a plated film of uneven exterior appearances. On the otherhand, isoleucine, phenylalanine and tryptophan are unsuitable for thepresent invention, because of their very low solubility in water.

The palladium complex is preferably formed by dispersing, in a solvent,at least one palladium salt selected from the group consisting ofpalladium chloride, palladium sulfate, palladium nitrate, palladiumhydroxide and palladium oxide, and the neutral amino acid describedabove, because the palladium salt can stably form the palladium complex.

The metal salt to be alloyed with palladium is preferably gold, silveror copper salt. It may be a sulfite, chloride, hydroxide or the like ofthese metals. More specifically, the gold salt may be at least one thesulfite, chloride and hydroxide. The silver salt may be the nitrate orthiosulfate, and the copper salt may be the sulfate.

The neutral amino acid may be in the form of alkaline metal salt, e.g.,sodium or potassium salt. The solvent is preferably water.

The plating solution of palladium alloy of the present inventionpreferably contains the neutral amino acid, palladium and metal salteach at a following concentration.

The neutral amino acid is preferably present at a concentration of 0.1to 2.0 mols/L in the plating solution, at which it can forms the complexto stabilize palladium and thereby to suppress precipitation or thelike. At below 0.1 mols/L, the complex may not be sufficiently formed.At above 2.0 mols/L, on the other hand, it may not be completelydissolved to separate out the precipitate.

Palladium is preferably present at a concentration of 1 to 30 g/L as Pd.At below 1 g/L, current density may be excessive for the palladiumconcentration during the plating work, possibly causing local formationof the plated film having rough surfaces. At above 30 g/L, on the otherhand, it may not be completely dissolved to separate out theprecipitate.

The metal salt is preferably present at a concentration of 0.1 to 30 g/Las the metal. At below 0.1 g/L, it tends to be difficult to give aneutectoid composition with palladium. At above 30 g/L, on the otherhand, it may not be completely dissolved to separate out theprecipitate.

The plating solution of palladium alloy of the present invention may beincorporated with a stabilizer or a buffering agent, in addition to thepalladium complex and the metal salt.

The stabilizer is preferably at least one species selected from sodiumsulfite, potassium sulfite and ammonium sulfite to give a platingsolution of high stability. It is preferably present at a concentrationof 1 to 200 g/L in the plating solution. At below 1 g/L, it tends to bedifficult to sufficiently stabilize the plating solution. At above 200g/L, on the other hand, it may not be completely dissolved to separateout the precipitate.

The buffering agent is preferably at least one species selected frominorganic salts, e.g., phosphate, borate, carbonate and so forth, andorganic carboxylic acids, e.g., lactic acid, citric acid, malonic acid,malic acid and so forth. A buffering agent works to stabilize a pH levelof the plating solution, and hence stabilize the plating solution. Thebuffering agent is preferably present at a concentration of 0.1 to 200g/L in the plating solution. At below 1 g/L, it tends to exhibit aninsufficient effect of stably keeping the pH level. At above 200 g/L, onthe other hand, it may not be completely dissolved to separate out theprecipitate.

A plating method with the plating solution of palladium alloy of thepresent invention is preferably carried out under the conditions of pH:6 to 10, temperature: 30 to 80° C. and current density: 0.1 to 7.0A/dm². At a pH below 6, the plated film formed may be cracked. At a pHabove 10 on the other hand, the plated film tends to have burnedsurfaces. At a solution temperature lower than 30° C., palladium may notbe coprecipitated with the metal. At higher than 80° C., on the otherhand, the plating solution tends to be unstable. At a current densitybelow 0.1 A/dm², the precipitate may have uneven exterior appearances.At above 7 A/dm², on the other hand, the precipitate tends to haveburned surfaces. The set current density varies depending on platingconditions, e.g., whether the solution is stirred or not.

A plated film formed by the above plating method preferably has apalladium eutectoid rate of 40 to 90% by weight. The film having therate in the above range will have good hydrogen permeability anddurability, when used for a hydrogen permeable membrane. The plated filmcan have a palladium eutectoid rate of 40 to 90% by weight, when it isformed under the conditions of amino acid concentration in the platingsolution: 0.1 to 1.0 mol/L, palladium concentration in the platingsolution: 5 to 30 g/L, metal concentration in the metal salt: 0.1 to 30g/L, solution pH: 6 to 10, temperature: 30 to 80° C., current density:0.1 to 7.0 A/dm².

The plated film formed by the above plating method is suitable forhydrogen separation membranes, because of its high hydrogen permeabilityand durability.

As discussed above, the plating solution of palladium alloy of thepresent invention is highly stable to form a plated film of uniformexterior appearances. It can give a plated film having a palladiumeutectoid rate of 40 to 90% by weight, and is suitable for hydrogenseparation membranes. The plated film can be also used for decorativepurposes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention are described below.

Examples 1 to 5

These examples used respective neutral amino acids of serine, valine,alanine, glycine and leucine as ligands for palladium complexes. Theplating treatment was carried out with plating solution compositions andunder conditions shown in Table 1.

TABLE 1 Solution composition Concentration Example 1 Palladium saltPalladium chloride (Pd concentration) 10 g/L Gold salt Sodium goldsulfite (Au concentration) 1 g/L Neutral amino acid DL-serine 1 mol/LStabilizer Sodium sulfite 50 g/L Buffering agent Disodium hydrogen 40g/L phosphate Plating conditions . . . pH 9, 60° C., 1 A/dm² Example 2Palladium salt Palladium chloride (Pd concentration) 5 g/L Gold saltSodium gold sulfite (Au concentration) 1 g/L Neutral amino acidDL-valine 1 mol/L Stabilizer Sodium sulfite 50 g/L Buffering agentsodium carbonate 50 g/L Plating conditions . . . pH 10, 60° C., 1 A/dm²Example 3 Palladium salt Palladium chloride (Pd concentration) 5 g/LGold salt Sodium gold sulfite (Au concentration) 1 g/L Neutral aminoacid DL-alanine 0.5 mol/L Stabilizer Sodium sulfite 50 g/L Bufferingagent Disodium hydrogen 50 g/L phosphate Plating conditions . . . pH 10,60° C., 3 A/dm² Example 4 Palladium salt Palladium chloride (Pdconcentration) 10 g/L Gold salt Sodium gold sulfite (Au concentration) 5g/L Neutral amino acid DL-glycine 0.5 mol/L Stabilizer Sodium sulfite 50g/L Buffering agent Disodium tetraborate + hydrate 50 g/L Platingconditions . . . pH 9, 50° C., 1 A/dm² Example 5 Palladium saltPalladium sulfate (Pd concentration) 5 g/L Gold salt Sodium gold sulfite(Au concentration) 1 g/L Neutral amino acid DL-leucine 0.5 mol/LStabilizer Sodium sulfite 100 g/L Buffering agent Trisodium citratedihydrate 50 g/L Plating conditions . . . pH 7.5, 60° C., 1 A/dm²

Conventional Example

The plating treatment was carried out using diamine dichloropalladium asa palladium salt with a plating solution composition and underconditions shown in Table 2.

TABLE 2 Solution composition Concentration Conventional Palladium saltDiamine dichloropalladium (Pd concentration) 1.8 g/L  Example Gold saltSodium gold sulfite (Au concentration) 10 g/L Ammonium sulfate 15 g/LPlating conditions . . . pH 8.2, 55° C., 0.5 A/dm²

Examples 6 and 7

These examples used a silver salt and a copper salt as a metal salt,respectively, and the plating treatment was carried out with platingsolution compositions and under conditions shown in Table 3.

TABLE 3 Solution composition Concentration Example 6 Palladium saltPalladium chloride (Pd concentration) 10 g/L Silver salt Silver nitrate(Ag concentration) 0.1 g/L Neutral amino acid Asparagine 0.5 mol/LStabilizer Sodium thiosulfate 50 g/L Buffering agent Dipotassiumhydrogen 50 g/L phosphate Plating conditions . . . pH 70, 60° C., 1A/dm² Example 7 Palladium salt Palladium chloride (Pd concentration) 10g/L Copper salt Copper nitrate (Cu concentration) 1.0 g/L Neutral aminoacid Asparagine 0.5 mol/L Stabilizer Dipotassium citrate 100 g/LBuffering agent Dipotassium hydrogen 50 g/L phosphate Plating conditions. . . pH 7.0, 60° C., 1 A/dm²

On completion of the plating treatment, each of the thin films of alloyformed was visually observed for the external appearance, and analyzedby ICP emission spectroscopy for eutectoid rate (% by weight) of thepalladium and metal ions in the plated film dissolved in aqua regalis.The results are given in Tables 4 and 5.

TABLE 4 Eutectoid rate (% by weight) External Appearance* Pd Au Example1 ◯ 75 25 Example 2 ◯ 80 20 Example 3 ◯ 90 10 Example 4 ◯ 60 40 Example5 ◯ 85 15 Conventional X 20 80 Example *◯: Uniform, good externalappearances, X: Irregularities observed on the surface

As shown in Table 4, the alloy film formed in each of Examples 1 to 5had good quality with uniform external appearances. For a palladiumeutectoid rate in the plated film, it was within a range from 40 to 90%by weight to give the excellent Pd—Au alloy film. On the other hand, theplating solution used in Comparative Example evolved precipitates duringthe plating work, and gave the uneven plated film with surfaceirregularities. Moreover, the plated film had a palladium eutectoid rateof 20% by weight.

TABLE 5 External Eutectoid rate (% b weight) Appearance* Pd Ag or CuExample 6 ◯ 80 20 Example 7 ◯ 60 40 *◯: Uniform, good externalappearances, X: Irregularities observed on the surface

As shown in Table 5, the alloy film formed in each of Examples 6 and 7which used a silver nitrate and a copper nitrate as a metal salt,respectively, had good quality with uniform external appearances. For apalladium eutectoid rate, it was within a range from 40 to 90% by weightto give the excellent Pd—Ag or Pd—Cu alloy film.

1. A plating solution of palladium alloy comprising a palladium complexand a metal salt, wherein the palladium complex is coordinated with atleast one neutral amino acid selected from the group consisting ofglycine, alanine, valine, leucine, serine, threonine, asparagine,glutamine and tyrosine as a ligand.
 2. The plating solution of palladiumalloy according to claim 1, wherein the palladium complex is formed bydispersing, in a solvent, at least one palladium salt selected from thegroup consisting of palladium chloride, palladium sulfate, palladiumnitrate, palladium hydroxide and palladium oxide, and the neutral aminoacid.
 3. The plating solution of palladium alloy according to claim 1,wherein the metal salt is one of gold salt, silver salt and copper salt.4. The plating solution of palladium alloy according to claim 1, whereinthe neutral amino acid is present at 0.1 to 2.0 mol/L.
 5. The platingsolution of palladium alloy according to claim 1, wherein palladium ispresent at a concentration of 1 to 30 g/L as Pd.
 6. The plating solutionof palladium alloy according to claim 1, wherein the metal salt ispresent at a concentration of 0.1 to 30 g/L as the metal.
 7. A palladiumalloy plating method comprising forming a plated film with the platingsolution of palladium alloy according to claim 1, carried out under theconditions of pH: 6 to 10, temperature: 30 to 80° C. and currentdensity: 0.1 to 7.0 A/dm².
 8. A hydrogen separation membrane produced bythe palladium alloy plating method according to claim
 7. 9. The platingsolution of palladium alloy according to claim 2, wherein the metal saltis one of gold salt, silver salt and copper salt.
 10. The platingsolution of palladium alloy according to claim 2, wherein the neutralamino acid is present at 0.1 to 2.0 mol/L.
 11. The plating solution ofpalladium alloy according to claim 3, wherein the neutral amino acid ispresent at 0.1 to 2.0 mol/L.
 12. The plating solution of palladium alloyaccording to claim 9, wherein the neutral amino acid is present at 0.1to 2.0 mol/L.
 13. The plating solution of palladium alloy according toclaim 2, wherein palladium is present at a concentration of 1 to 30 g/Las Pd.
 14. The plating solution of palladium alloy according to claim 3,wherein palladium is present at a concentration of 1 to 30 g/L as Pd.15. The plating solution of palladium alloy according to claim 9,wherein palladium is present at a concentration of 1 to 30 g/L as Pd.16. The plating solution of palladium alloy according to claim 4,wherein palladium is present at a concentration of 1 to 30 g/L as Pd.17. The plating solution of palladium alloy according to claim 10,wherein palladium is present at a concentration of 1 to 30 g/L as Pd.19. The plating solution of palladium alloy according to claim 11,wherein palladium is present at a concentration of 1 to 30 g/L as Pd.20. The plating solution of palladium alloy according to claim 12,wherein palladium is present at a concentration of 1 to 30 g/L as Pd.